In recent years, demands for drawn and ironed cans, or so called DI cans have been growing remarkably. Largely because of seam-free and aesthetically improved features, DI cans have been extensively used-for canning beer, juices and other beverages.
DI cans are produced commercially on a mass production scale and DI can manufacturing processes generally include blanking and drawing metal strips into shallow cups, redrawing and ironing the cups to form hollow tubular bodies with thin sidewalls, and trimming the open ends of the tubular bodies to a predetermined height. Then, the trimmed bodies are subjected to surface treatment processes, in which sprays of treatment liquid such as degreasing solutions, industrial water, chemical solutions and deionized water are directed against the inner and outer surfaces of the trimmed bodies. Subsequently, the bodies are dried in a drying oven, decorated externally, coated internally with a protective coating and finally subjected to necking and flanging and formed into complete can bodies.
A line of production equipment to perform the above processes and manufacture DI cans is typically very long and many can manufacturers have been experiencing difficulties in accommodating such a long line in their available space. Various efforts have so far been made to develop compact lines by making component machines of the equipment more compact and, for example, a device for the surface treatment, which essentially occupies the largest installation space among components of the line equipment, has ordinarily been designed to accommodate a drying oven in a piece of machinery for continuous processes.
One of the most extensively adopted systems for the surface treatment in the industry uses an endless mesh conveyor belt having large numbers of openings that allow passage of sprays of the treatment liquid, and the conveyor belt progresses through a pre-wash zone, a treatment zone and a post-wash zone accommodated in a long tunnel and partitioned one from another, so that trimmed can bodies placed in a mass in an inverted position with their bottoms up off the conveyor belt receive sprays of the treatment liquid directed from a series of spray nozzles positioned above and beneath the upper flight of the conveyor belt (U.S. Pat. No. 3,952,698).
Nowadays, DI cans having extremely thin sidewalls or so called lightweight DI cans have become available in the industry as the result of efforts of various manufacturers for savings of manufacturing costs. Since these cans are very light, however, they can be readily tilted or displaced to come into contact with another on the conveyor belt or tipped over by impingements of sprays during the surface treatment, and such can-to-can contacts and tipping over often result in defects such as poor and irregular wash and inadequate surface finish. Such defects may adversely affect adhesion performance and corrosion resistance of a film of the protective coating and extremely deteriorate luster of the coated or decorated surfaces to such an extent that commercial values of finished cans may be completely destroyed.
U.S. Pat. No. 3,291,143 discloses an apparatus for surface treatment of lightweight cans as illustrated in FIG. 8 (a side sectional view of the apparatus) and FIG. 9 (a sectional view taken along line IX--IX in FIG. 8). The apparatus comprises a surface treatment housing 15, a lower endless conveyor belt 11 which progresses with cans K held thereon through the housing, a plurality of lower nozzles 13 disposed beneath the lower conveyor belt 11, a plurality of upper nozzles 14 disposed above the cans K in the housing and arranged to face the lower nozzles 13, and an upper endless mesh conveyor belt 12 surrounding the upper nozzles 13 and progressing in the same direction as the lower conveyor belt 11. The specification further describes that the lower flight 12a of the upper conveyor belt 12 should preferably be spaced upwardly by about 0.3 to 0.6 cm (i.e., 1/8 to 1/4 inches) from the bottoms of the cans K held in the inverted state on the lower mesh conveyor belt 11 and fed continuously in the direction of the arrow Z.
As cans K travel through the housing, they receive sprays of the treatment liquid directed from the upper and lower nozzles 13 and 14. The spray pressure of the lower nozzles is set so as to overcome that of the upper nozzles to urge the cans upwardly against the lower flight of the upper conveyor belt 12, and with this arrangement, it is indicated that even light weight cans may not be tilted or displaced to come into contact with one another or tipped over during the surface treatment.
From the viewpoint of productivity in a mass production, the apparatus disclosed in U.S. Pat. No. 3,952,698 is certainly desirable as the mesh conveyor belt of the apparatus for holding cans has no partitioning and thus permits a large number of cans to be placed on it. With such apparatus, however, cans on the conveyor belt may come into contact with one another during the processes so that contacting portions and adjacent areas of the cans may not receive adequate sprays.
Since the upwardly and downwardly directed sprays in the apparatus will not prevent contact of cans, occasional occurrence of defects due to can-to-can contacts is unavoidable with such apparatus. It should be noted that, in such apparatus, sprays of the treatment liquid just flow through gaps between adjacent can bodies, so that when a can has just advanced past the sprays a negative pressure is created momentarily in the gaps to pull an adjacent can, causing can-to-can contacts and resultant defects.
Further, varied flow of cans into such apparatus may cause additional problems. Depending on the flow of cans, they may be pushed by one another and forced to slide over the surface of the conveyor belt, so that sidewall portions near the bottom rim of a can are rubbed with those of another to develop a band of dark scars in the rubbed portions and nicks are caused at the edge of the open end due to friction with the conveyor belt. Also, if a can is pushed excessively, it may jump out of the way or tip over. On the other hand, the apparatus disclosed in the U.S. Pat. No. 3,291,143 permits efficient washing of the inner and outer surfaces of lightweight cans by relatively high fluid pressure of sprays directed thereto as the cans are held against the lower flight of the upper conveyor by the pressure of the upwardly directed sprays. Since fluid pressures created in the lateral directions by the sprays are not controlled in such apparatus, however, the cans may be moved in the lateral directions due to imbalanced spray pressure and brought into contact with one another to cause defects, particularly when the cans are closely spaced from one another in an attempt to improve productivity. In the above apparatus, lateral forces of upwardly and downwardly directed sprays are not balanced as the upper and lower sprays are not aligned with each other.
As discussed above, neither of the aforementioned prior art surface treatment apparatus has adequate measures for eliminating can-to-can contacts and resultant defects as well as certain incidental damage to drawn and ironed lightweight cans.